1. Field of the Invention
The present invention relates to a center pivot irrigation line system having a drive apparatus for providing proper driving of the irrigation line over uneven terrain, and more particularly to such a system in which the irrigation line is driven at a rate dependent upon the magnitude of water pressure at a selected location in the line, and hence dependent upon the relative elevation of the line.
2. Description of the Prior Art
In a conventional center pivot irrigation system, an elongated irrigation line is supported at intervals by towers on wheels. At least one of such towers, usually the outermost tower, contains a prime or lead drive motor for moving this tower. The remaining towers each have drive motors operable to move them about the pivot in substantial alignment with the motion of the outermost tower. In one conventional system, the drive motor comprises an electrical induction motor which is energized by a percentage timer for a preselected subinterval of each minute so that the irrigation line travels at a uniform average rate about the pivot. However, when this system travels uphill, water pressure in the line decreases. As a result, the volume of water distributed from sprinklers spaced along the line diminishes with increased elevations in the line. Conversely, as the line moves downhill, water pressure in the line increases thereby increasing the volume of water distributed from the sprinklers. Consequently, with a uniform rate of travel, locations at lower elevations tend to be overwatered because of the relatively high line water pressures, while those at higher elevations are underwatered because of the relatively low line water pressure. Not only does this uneven irrigation hinder the uniform growth of vegetation in the irrigated area, water tends to form ponds in downhill regions, which can mire the irrigation line wheels and entirely stop its operation.
In an attempt to alleviate this problem, a pressure regulator or flow control device has been installed at each sprinkler head along a line to promote even distribution of water from the heads. However, there are typically up to 100 or more sprinkler heads along each such line so that, a large number of these devices are required. These devices are costly when installed in quantity and have limited life because of pressure caused deterioration of rubber valve materials typically included within them.
Another common prior art device comprises a programmed irrigation system which senses the angular position of the line about its pivot. A stored rate of travel instruction for that particular angular position controls the speed of the line. Similar programmed instructions are provided for each angular position of the line about its pivot. These devices are extremely costly and suffer from the added drawback that they must be custom programmed for each installation. Typically, such programming is not within the skill of a ranch hand or other individual who operates the line. Therefore, highly paid technicians must be transported to the installation site to program such a system for operation and to maintain it.